The present invention generally relates to shimming a magnetic resonance imaging (MRI) magnet and more particularly is concerned with a method for graphical visualization of a measured magnetic field on an imaging volume of a MRI magnet.
A MRI magnet generates a magnetic field with a generally spherical region of high homogeneity typically referred to as the imaging volume or DSV of the magnet. The magnetic field is typically measured on this spherical region by sampling the magnetic field at points, typically equidistantly spaced apart, along its latitude and longitude. Optionally, some additional points along its axis may also be sampled.
Traditionally, the magnetic field on a DSV is represented as a list of field points in ASCII, or as a table of the spherical harmonic content of the field on the surface of the sphere (and thus also in the interior for a volume free of magnetic material). This representation, although useful for computer optimization and interpretation by codes, is not very useful in visually showing inhomogeneous regions on the DSV, or shim errors caused by misplaced shims.
Consequently, a need exists for an innovation which will provide a means to visually assess to the condition of the magnetic field and its inhomogeneities.
The present invention provides a method for graphical visualization of a measured magnetic field on an imaging volume of a MRI magnet which is designed to satisfy the aforementioned need. The method of the present invention allows the graphical representation of the measured magnetic field in either a two-dimensional or three-dimensional format and the direct visualization of field inhomogeneities through surface texture and/or coloring schemes. In such manner, the method greatly enhances understanding of the origin of field disturbances.
In one embodiment of the present invention, a method for graphical visualization of a measured magnetic field on an imaging volume of a MRI magnet is provided which comprises the steps of providing data values measured at selected points of a measured magnetic field on an imaging volume of a MRI magnet, converting each of the data values to a visualizable value based on where the data value lies within a preset range of data values and thus within a range of visualizable values corresponding to the range of data values, and plotting each visualizable value corresponding to each data value at a position on a grid corresponding to the position of the selected point of the measured magnetic field on the imaging volume such that the visualizable values once plotted on the grid provide a graphical visualization of the condition of the measured magnetic field on the imaging volume of the MRI magnet.
The grid can be either of planar configuration or spherical configuration. In the spherical configuration of the grid, the method further comprises the step of rotating the spherical grid to highlight selected areas of the magnetic field.